Yamaha YM2203

The YM2203, aka OPN (FM Operator Type-N), is a three-channel sound chip developed by Yamaha. It's the progenitor of Yamaha's OPN family of FM synthesis chips used in many videogame and computer systems throughout the 1980s and early 1990s. The YM2203 itself was used in a variety of NEC computers, along with various arcade game machines.

The YM2203 and the rest of the OPN synthesizer family generate sound via frequency-modulated digital sine waves. It included 12 operator "cells", each generating a 13-bit sine wave at a programmable frequency, the volume of which is controlled by a programmable ADSR envelope generator. The output of these cells could be either summed together by the mixer, or fed into the input of another cell, in 4-cell batches creating the final sound values or "channels". 4 operator cells per channel allowed a total of 8 different permutations of cell connections, known as "algorithms" or instrument patches.

The number of channels varied across the OPN family: 3 in the OPN; 4 in the YM2608 OPNB; and 6 in the YM2608 OPNA, YM2612 OPNC, and various derivatives. The resulting digital sound output of each channel through the mixer was then converted to analog sound through a DAC, usually with an external companion chip but with an internal DAC in the case of the YM2612, 3438, and related budget models.